Recently, a multiple input multiple output (MIMO) system has received much attention as a broadband wireless communication technology. The MIMO system means a system that enhances communication efficiency of data by using a plurality of antennas. The MIMO system may be divided into a spatial multiplexing scheme and a spatial diversity scheme depending on transmission of same data.
The spatial multiplexing scheme means a scheme that may transmit data at high rate without even increasing a system bandwidth by simultaneously transmitting different data through a plurality of transmitting antennas. The spatial multiplexing scheme means a scheme that may obtain transmission diversity by transmitting same data from a plurality of transmitting antennas. An example of the spatial diversity scheme includes a space time channel coding scheme.
Also, the MIMO system may be divided into an open loop system and a closed loop system depending on feedback of channel information from a receiving side to a transmitting side. An example of the open loop system includes a space-time trellis code (STTC) system in which a transmitting side transmits information in parallel while a receiving side detects a signal by repeatedly using a zero forcing (ZF) scheme and a minimum mean square error (MMSE) scheme and obtains transmission diversity and encoding gain by using a blast and spatial area that can increase information as much as the number of transmitting antennas. An example of the closed loop system includes a transmit antenna array (TxAA) system.
A coordinated multi-point system (hereinafter, referred to as ‘CoMP system’) is the system for improving throughput of a user located at the cell edge by applying improved MIMO transmission under the multi-cell environment. If the CoMP system is applied, inter-cell interference may be reduced under the multi-cell environment. In case of the CoMP system, a user equipment may commonly be supported with data from multi-cell base stations.
Also, each base station may improve system throughput by supporting the same radio frequency resource to one or more user equipments UE1, UE 2, . . . , UE K at the same time. Also, the base station may perform a space division multiple access (SDMA) method based on channel state information (CSI) between the base station and the user equipment.
The CoMP system may be divided into a coordinated MIMO (Co-MIMO) type joint processing (JP) scheme through data sharing and a coordinated scheduling scheme/beamforming (CS/CB) scheme.
FIG. 1 is a conceptional diagram illustrating CoMP operations of intra base stations (eNBs) and an inter base station (eNB) according to the related art.
Referring to FIG. 1, intra base stations 110, 120 and an inter base station 130 exist under a multi cell environment. According to a long term evolution (LTE) system, the intra base stations include several cells (or sectors). Cells that belong to a base station to which a specific user equipment belongs are in the relation of intra base stations 110, 120 with the specific user equipment. Namely, cells that share the same base station as that of a cell to which a user equipment belongs are those corresponding to the intra base stations 110 and 120 while cells that belong to other base stations are those corresponding to the inter base station 130. In this way, although cells based on the same base station as that of the specific user equipment transmit and receive information (for example, data and channel state information (CSI)) through x2 interface, cells based on a base station different from that of the specific user equipment may transmit and receive information through a backhaul 140.
As illustrated in FIG. 1, a single cell MIMO user 150 located within a single cell may perform communication with a single serving base station in one cell (sector), and a multi-cell MIMO user 160 located at the cell edge may perform communication with a plurality of serving base stations in multiple cells (sectors).
However, since each cell generates a reference signal sequence based on each cell ID (identifier), the user equipment cannot estimate this value desirably in case of a multi-cell based CoMP operation. In this respect, a new method for solving this problem will be required.